A nuclear fuel assembly which finishes combustion in a terminal phase of a nuclear fuel cycle and can not be used is called as a spent nuclear fuel. Since the spent nuclear fuel contains a radioactive material such as an FP (fission product) or the like, it is necessary to thermally cool, so that the spent nuclear fuel is cooled by a cooling pit in a nuclear power plant for a predetermined period (one to three years). Thereafter, the spent nuclear fuel is received in a cask corresponding to a shielded vessel, and transported to a reprocessing facility by a truck or the like so as to be stocked. When the spent fuel assembly is received within the cask, a holding element having a grid-like cross section called as a basket is used. The spent fuel assemblies are inserted in a plurality of cells corresponding to receiving spaces formed in the basket one by one, whereby it is possible to secure a proper holding force against a vibration during the transportation or the like.
As a conventional example of the cask mentioned above, various kinds of structures are disclosed in “Nuclear Power eye” (issued in Apr. 1, 1998 by DAILY INDUSTRIAL PUBLICATION PRODUCTION), Japanese Patent Application Laid-Open No. 62-242725 and the like. A description will be given below of a cask corresponding to a base which develops the present invention. In this case, the following contents will be shown for a convenience of description, and does not mean so-called known and used techniques.
FIG. 24 is a perspective view which shows one example of a cask. FIG. 25 is a cross sectional view in an axial direction of the cask shown in FIG. 24. A cask 500 is constituted by a cylindrical barrel main body 501, a resin 502 corresponding to a neutron shield provided in an outer periphery of the barrel main body 501, an external cylinder 503, a bottom section 504 and a cover section 505. The barrel main body 501 and the bottom section 504 are formed by a forged product made of a carbon steel corresponding to a γ ray shield. Further, the cover section 505 is constituted by a primary cover 506 and a secondary cover 507 which are made of a stainless steel or the like. The barrel main body 501 and the bottom section 504 are connected according to a butt welding. The primary cover 506 and the secondary cover 507 are fixed to the barrel main body 501 by a bolt made of a stainless steel or the like. A metal O-ring is interposed between the cover section 505 and the barrel main body 501, whereby an air tightness in an inner section is kept.
A plurality of inner fins 508 executing a thermal conduction are provided between the barrel main body 501 and the external cylinder 503. The inner fins 508 employ a copper material which increases a thermal conduction efficiency. The resin 502 is poured into a space formed by the inner fins 508 in a flowing state and is solidified due to a cooling operation. A basket 509 is structured such that sixty nine square pipes 510 are collected in a bundle shape as shown in FIG. 24, and is inserted within a cavity 511 of the barrel main body 501 in a substantially bound state.
The square pipes 510 are made of an aluminum alloy in which a neutron absorber (boron: B) is mixed so as to prevent the inserted spent fuel assemblies from reaching a critical state. In this case, trunnions 513 which suspend the cask 500 are provided in both sides of the cask main body 512 (one is omitted). Further, a buffer 514 in which a wood material or the like is assembled in an inner section so as to constitute the buffer material are mounted to both end sections of the cask main body 512 (one is omitted).
In this case, the basket 509 may employ a structure formed in a box-of-cake shape, or an integrally cast structure in addition to the structure in which the square pipes 510 are collected in the bundle shape. The box-of-cake shaped basket is constructed by forming notches in both sides of a rectangular plate basket material and vertically crossing the basket materials by the notches so as to be alternately assembled. Accordingly, it is possible to form the basket having a plurality of cells. Further, the basket having the integrally cast structure is constructed by forming a whole of the basket according to a casting, and cells thereof are molded by using a core or according to a machining.
In the instance of actually manufacturing the cask 500 mentioned above, it is normally necessary to consider design conditions such as a receiving number, a size, a weight and the like of the spent fuel assemblies. In particular, it is preferable to employ a cask in which a receiving number is large, an outer diameter is small and a weight is small. However, according to the structure of the cask 500 mentioned above, since the square pipe 510 in an outermost periphery is in line contact with the inner surface of the cavity 511 (this matter is applied to both of the box-of-cake shaped basket and the basket having the integrally cast structure in the same manner), a space S is generated between the basket 509 and the cavity 511, and a heat conduction from the cell 515 to the barrel main body 501 cannot be efficiently executed. Further, since the diameter of the barrel main body 501 is increased due to an existence of the space S, the cask 500 becomes heavy.
On the contrary, since an amount of radiation leaking out of the cask is restricted by a total amount of the neutrons and the γ rays, it is sufficient to reduce a thickness of the barrel main body 501 in order to lighten the cask 500. However, since it is necessary to constitute the γ rays shield, a thickness which secures a γ ray shielding function is required in a side of the barrel main body 501. Further, the cask 500 mentioned above is structured such as to be capable of receiving sixty nine fuel assemblies which have never been achieved by the conventional art, however, when the diameter of the barrel main body 501 is reduced in the structure for the purpose of achieving a predetermined weight, the receiving number of the spent fuel assemblies is reduced.